The government's charter school legislation will break labour, human rights and free trade provisions, says the Council of Trade Unions (CTU).
Meanwhile, the chief ombudsman has told Parliament's Education and Workforce Select Committee the schools should not be exempt from the Official Information Act, while the Association of Educational Research has warned evidence from around the world showed charter schools were not effective.
The committee was hearing submissions on the bill reintroducing the publicly-funded, private schools.
CTU secretary Erin Polaczuk said a regulatory impact statement showed a late addition to the bill blocking charter staff from joining multi-employer collective agreements would breach multiple agreements.
"The regulatory impact statement notes quite rightly that, quote, 'There is a strong misalignment with the ILO (International Labour Organisation) as it creates a significant restriction on parties ability to bargain freely. If a complaint were to be brought to the ILO, it is likely that the ILO would confirm the misalignment'," she said.
"The Ministry of Education is using very clear language telling the government that they are proposing in this bill to break International Labour Organisation conventions."
Polaczuk said the statement also said the bill would likely breach elements of free trade agreements with the UK and EU.
She said the bill would also prevent state school teachers from refusing to provide services to charter schools but ministry advice indicated that would breach ILO agreements and the Bill of Rights Act.
Union legal advisor Gayaal Iddamalgoda told the committee the advice was later redacted from the impact statement on the basis that it was legally privileged.
'The Official Information Act must cover charter schools'
Later in the select committee hearing, Chief Ombudsman Peter Boshier said the the bill was wrong to exclude charters from the Official Information Act (OIA).
He said the issue was of constitutional importance.
"The Official Information Act must, in my submission, cover charter schools. If these schools are excluded, as proposed, from the act they'll be less accountable to students, parents and taxpayers than schools in the state system," he said.
Boshier said the public had a right to expect transparency over the use of public funds and families should have the same rights to information at charters as at state schools.
"Being subject to the OIA ensures these bodies are accountable to taxpayers for their performance," he said.
Former charter school head Alwyn Poole told the committee he agreed with Boshier.
Poole said he planned to submit proposals for four charter schools and refuted other submitters' suggestions that charter teachers would be worse paid than teachers in state schools.
'A fruitless policy'
The Association for Research in Education told the committee charter schools were not needed.
Association spokesperson Bronwyn Wood said New Zealand schools already had a lot of autonomy and increasing school choice would not help.
"Choice is incredibly appealing. It appeals to countries all over the world - that choice would actually be the way in which we could solve many of the educational ails [sic]. Unfortunately, most of the evidence is that choice increases choices for the well-to-do and decreases choices for those that already had less, with growing inequalities. And this charter bill will feed into that completely," she said.
"Charter schools are not needed. We already have high, high flexibility in which almost everything that charter schools propose could happen except that you can't currently teach without a registration and you can't ignore the New Zealand curriculum."
Wood said charters were a fruitless policy.
She said Sweden had 30 years experience with a type of school similar to charters, but in March this year declared the schools were a complete disaster.
Teacher Ann Kendon told the committee it seemed irresponsible to spend so much money on a new type of school that New Zealand did not need.
"I've never in my classroom thought: 'I really could do with a sponsor at this school'," she said.
Kendon told members she did not believe the schools would get better results for children than state schools, and if they did they were likely to be bending their figures.
Teacher Philip Kendon told the committee the state system encouraged collaboration between teachers and that helped them to do a good job.
He said New Zealand's state school system could be improved, but it worked and charters were not needed.
"Please listen to the experts, those of us who have devoted our lives to teaching in the state sector. Please don't do this," he said.